The antigenic constitution of 2 strains and 6 cloned Trypanosoma cruzi stocks is being analysed immunoelectrophoretically. Five to 7 of the antigens detected were common to all stocks tested. However, 1 to 2 stock-specific antigens were also found, demonstrating interclonal antigenic differences within a T. cruzi strain, i.e., that the population of organisms which constitute a T. cruzi strain is antigenically heterogenous.